Baluns are widely used in electrical and electronic engineering for the purpose of converting a balanced input to an unbalanced output or vice versa. In radio frequency (RF) and microwave monolithic integrated circuits (MMICs), baluns are usually used for designing, for example, push-pull low-noise amplifiers (LNA) or double balanced mixers. The balun can be one of the most critical components in determining the circuit's overall performances. For example, it enables high linearity performance for the push-pull configured amplifiers. Furthermore, numerous modern electronic systems demand baluns that can operate over a substantially wide frequency band for both military and commercial applications. Most reported MMIC wideband baluns, however, are of octave bandwidth or less with high insertion loss.
Typical designs involving baluns having primary and secondary windings that are configured on the same plane. Although operable, these planar balun designs have very limited bandwidth due to low self-resonant frequency or large parasitic capacitance. Implementation of a true wideband balun needs to overcome many technical obstacles. For example, designing baluns in regular MMIC processing is often quite limited due to the low self-resonance frequency (SRF) as well as low quality factor (Q) of the balun windings. Another difficulty lies in the fact that it is difficult to extend the operation band to a substantially lower frequency, since it requires more turns of winding. The increased number of turns introduces much unwanted parasitic capacitance, which in turn dramatically lowers the self-resonant frequency of a balun. As a result, the operation bandwidth is limited.